1. Field of the Invention
This invention relates to electron beam-generating devices, and more particularly, it relates to a dual-mode electron gun especially suitable for traveling-wave tubes.
2. Description of the Prior Art
Dual-mode traveling-wave tubes have been developed in which a single tube is designed to operate selectively in either a low power mode or a high power mode. The power level of a traveling-wave tube is a function of both the current and voltage of the electron beam used to interact with the propagating electromagnetic waves. Hence, in order to achieve dual-mode operation, the beam current is selectively switched between different levels in a manner sufficiently compatible with other tube parameters such that desired operation in both modes may be obtained.
A classic form of dual-mode electron gun is described in U.S. Pat. No. 3,859,552 to Richard Hechtel, as well as in the Hechtel and Hamak paper, "A Dual Mode Electron Gun Having Non-Intercepting Grids", 1973 IEDM Technical Digest, pp. 171-174. In this electron gun, a beam of large cross-sectional area is emitted from the entire cathode surface in the high power mode, while a beam of reduced cross-sectional area, but of the same current density, is emitted from the central portion of the cathode surface in the low power mode. The foregoing is achieved by splitting the control grid of the gun into an inner circular grid and an outer annular grid. In order to generate the large cross-section beam, a positive voltage with respect to the cathode is applied to both control grids. The reduced cross-section beam is generated by making the voltage on the outer control grid negative with respect to the cathode. In order to eliminate current interception by the control grids, a shadow grid having the same geometry as the control grids and maintained at cathode potential is disposed between the cathode and the control grids.
During operation of the aforementioned electron gun in generating the reduced cross-section beam, an acceptable value of negative voltage applied to the outer control grid is unable to prevent emission from an annular region of the cathode immediately radially outwardly of the cathode region over which the inner control grid projects. Thus, a spurious annular beam portion is generated radially outwardly of the desired low mode beam. In addition, the electric field between the outer and inner control grids deflects the spurious beam portion radially inwardly. The spurious electrons eventually are intercepted either by downstream electrodes of the electron gun or by the slow-wave circuit of the traveling-wave tube in which the gun is utilized, thereby wasting beam current and reducing the operating efficiency of the tube.